A series of cloned T lymphocytes from C57B1/10 mice has been examined by two-dimensional gel electrophoresis to try to define the proteins which are responsible for the differentiated functions of these lymphocytes, especially cytotoxicity. By using 35S methionine-labeling, over two hundred proteins can be reproducively resolved from the total cell extracts. All of these except two are found in all the clones examined. The two spots which are variable are found in some clones which are cytotoxic and some clones which are not. Non-cytotoxic variants of clones which originally were cytotoxic have identical patterns of proteins to the cytotoxic parent clones. We thus conclude that proteins uniquely responsible for the differentiated functions of lymphocytes are not among the major proteins species synthesized by the cells.